It means no NM float tube, but if you have a quadrail you can have the sling swivel mounted on it. There is no other way to mount a sling swivel on the handguard unless it is a quad rail of some kind. This is a little golden nugget of knowing the rules. The resulting configuration is a float tube that has the same benefit legally as the illegal NM float tube. I don't think they realize that the quad rails are float handguards, or if they do they just want to eliminate all NM rifles as we know them to be constructed, but still allow the "as seen in Afghanistan" quad rail M4 shorties.

Is an M4 legal in CMP MM with a free float tube? I thought by the CMP definition, a free float tube was not on an M4. Maybe my misunderstanding lies in that.

I understand the harmonics as well as a rifle without a sling vs. one with. I would much rather use my MM rifle with a sling, but alas, it's a "stock" M&P and when I sling up tight, it's all over the place, unlike my NM AR. My other inventory choices are M1A's, which have match barrels (DQ's them from MM) and some stuff I haven't had adequate range time with (MAS, M42b, etc.).

Standard sling swivels are part of the front sight fixture that is pinned to the barrel. Sling tension on a standard front sight sling swivel applies pressure to the barrel. The reason people have problems with zeros with this is because they don't have the forward elbow under the rifle, and tension pulls it left, or inconsistent tension allows it to go side to side. This is also true for NM rifle shooters too. Any inconsistency will cause wandering zeros, and many will blame the lack of a float tube when in fact it could be anything that isn't consistent from shot to shot. Let me make this perfectly clear. You do not need a float tube to clean the prone slow, or rapid fire at 200 yards.
What you need is a good position with the forward elbow under the rifle. This is the hardest thing for people to learn on their own. Extend the forward arm, and set the elbow deep under the rifle as if you are trying to get your armpit on the deck. When this is done correctly, your upper arm will be mostly flat on the deck, and the weight is not on the point of the elbow, but on the side.
You don't need one but, a quad rail is a float tube. They are attached the same way as NM float tubes. A quad rail is a float tube without the handguard, because the handguard is the tube. The CMP isn't considering the quad rails as float tubes, based on the ruling that you can have your sling swivel attached to the handguard. This suggests that they are calling a quad rail a handguard, and it is.

Thanks Greg. I normally have a pretty good prone, left elbow underneath, right elbow in close. I just don't seem to get the consistency like I do with all my other "slung" rifles. So for now, whatever I'm doing wrong, I guess I'll let it go and continue to fire w/o a sling on this rifle.